1. Field of the Invention
This invention relates to a clamp circuit for use in an image pick-up device of a video camera or the like to clamp the black level of the output signal of the image pick-up to a predetermined electrical potential and, more particularly, to a clamp circuit, which clamps the optical black level signal of the output of an image pick-up. Clamping is accomplished in an ineffective picture area corresponding to the horizontal blanking interval for each horizontal scanning interval.
The output signal obtained from an image pick-up device is usually subjected to various signal processing with its black level as a reference. Meanwhile, it is well-known in the art that an image pick-up tube or a solid-state image sensor, which is used as the image pick-up section of the image pick-up device, is subject to dark current fluctuations due to causes such as ambient temperature changes. The dark current fluctuations cause variations of the output signal level.
To cope with this drawback, it has been proposed to provide the image pick-up section 1, as shown in FIG. 1, with an optical black-detecting portion 4 which is shielded on, a horizontal side against light in an ineffective picture area 3 surrounding an effective picture area 2 which corresponds to the horizontal blanking interval of the output signal of the image pickup section 1. It has also been proposed to clamp the optical black level of the image pick-up output signal to a predetermined electrical potential. The output signal of the image pick-up section 1 with the optical black-detecting portion 4, as shown in FIG. 2A, has a signal interval TH.sub.D which corresponds to the optical black-detecting portion 4. The signal interval TH.sub.D is within the horizontal blanking interval TH.sub.BLK of the horizontal scanning interval H. The signal level of the signal interval TH.sub.D depends on the optical black-detecting portion 4, which is shielded from light. That is, it does not depend on the image pick-up light intensity but is determined by the level of the dark current. Compensation of said optical black level for dark current fluctuations, therefore, can be obtained to maintain a predetermined electrical potential by clamping the signal level in said signal interval TH.sub.D, i.e., the optical black level, to a predetermined level with a clamp pulse, as shown in FIG. 2B, which occurs in said signal interval TH.sub.D during every horizontal scanning interval H.
In the solid-state image sensor consisting of a CCD (Charge Coupled Device) or the like, a light-receiving section corresponding to the picture elements is provided only for said effective picture area 2 and only a transfer resister for said ineffective picture area 3 is provided in order to obtain as many effective picture elements as possible. The output signal of the image pick-up obtained from the image pick-up section consisting of said solid-state image sensor, thus is subject to the influence of dark current only in the transfer register for the portion which corresponds to the ineffective picture area 3. The portion which corresponds to the effective picture area 2 is subject to the influence of dark currents in both the light-receiving section and the transfer register section. Particularly, a pronounced level variation occurs due to said dark current between the vertical blanking interval, during which no signal is read out, and the subsequent scanning interval.
Therefore, if the time constant of the clamp circuit is reduced so that it is possible to faithfully follow dark current changes in the solid-state image sensor output signal for each field or frame, low frequency noise components that are generated at the time of clamping will be increased which results in the formation of strips or noise for each line which extremely deteriorate the quality of reproduction. On the other hand, if the time constant of the clamp circuit is increased to reduce said low frequency noise components, sufficient compensation for the level variations generated between said vertical blanking interval and the subsequent scanning interval cannot be obtained, which has the disadvantage that flicker in the picture or brightness variations in the vertical direction of that picture result.